Varieties of organic composites are in use for their superior strength and low density compared to metallic structures. These organic composites consist of fibers embedded in polymer matrices. Many layers are superposed on each other to obtain high strength and superior properties. For several decades, the aerospace industries have taken the advantage of superior physical properties and weight of the composite materials. Generally, composites of carbon fibers with polymer matrix are used in aerospace applications. While the polymer composites provide excellent strength/weight ratio, their strength degrades dramatically when exposed to heat. On an aircraft, this could happen due to fire accidents or accidental exposure to excessive heat during repair or due to heat generation due to lightening strike.
Several experimental techniques have been used to evaluate the heat damage in composites. Some of these techniques are nondestructive in nature and some are destructive. While destructive techniques essentially attempt to measure the loss of mechanical strength in the composite, nondestructive evaluation (NDE) techniques attempt to relate the measured property to the loss of strength. In particular, NDE techniques provide information about the elastic modulus of the degraded material. However, significant change in the elastic modulus occurs only when gross damage occurs in the material. Accordingly, prior art NDE techniques are not sensitive to detecting “incipient” damage in composites which is responsible for the loss of physical or mechanical properties without gross changes in structure such as cracking, blistering or delamination.
For example, NDE of materials based on acoustic wave propagation depends on the interaction of acoustic waves with defects in the material. In particular, prior art acoustic based NDE methods use the returned acoustic energy to measure elastic properties of materials and to identify defect locations in the material based on the changes in the elastic properties caused by presence of the defects. In such prior art acoustic wave propagation systems, for the best NDE testing results, a piezoelectric transducer is either placed in contact with the material or both the transducer and material to be tested are immersed in water in order to launch sufficient acoustic wave energy into the material.
Another non-contact NDE method is infrared (IR) thermography. IR thermography is used to detect and image changes in the thermal property of materials. In this prior art method, a heat pulse is incident on the surface of the material to be tested. The heat diffuses into the material uniformly causing gradual temperature changes. An IR camera is used to image the changes in the temperature. IR images are acquired as a function of time from the initial excitation of the heat pulse. Presence of defects in the material alters the distribution of temperature. Analysis of the IR images can be used to detect and locate the defects in the material. However, it has been observed that significant changes in thermal properties occur only when the composite has undergone gross damage. Changes in thermal properties during early stages of heat damage in composites are quite small and hence the IR thermography is not a sensitive method.
Another NDE technique is the thermo-elastic measurements. This technique takes the advantage of the slight reduction of temperature of the material, when subjected to tension, while a small increase in temperature is observed when subjected to compressive stress. The thermo-elastic technique is used in evaluation of the stress distribution under load in materials and components. The temperature distribution is most often measured using a high sensitivity infrared camera. Following similar arguments and instead of using mechanical loading to create a distribution of temperature, high amplitude acoustic wave can be used. The temperature distribution can be visualized using IR camera.
Another NDE technique combines acoustic excitation and IR thermography to evaluate the heat damage in composite materials. In this technique, an ultrasonic horn is brought into contact with the composite and the structure is excited. The high amplitude vibration caused by the horn, create vibrations of different modes in the specimen. In the neighborhood of a defect (crack or delamination), the modes of vibration create extra heat due to friction between the two faces of the crack or other irreversible thermo-elastic effects. Temperature changes due to vibrations are captured using a high sensitivity IR camera. However, this contact technique of exciting the structure with high amplitude ultrasonic waves raises concerns about the damage that could be introduced due to excitation process via direct contact between the composite and the acoustic horn. The acoustic horn in contact when excited operates like a hammer and may cause damage to the specimen. In addition, the relation between the temperature changes and the amount of heat damage is complicated by excitation of many different modes of vibration in the structure due to direct contact between the horn and specimen. This methodology is known in the literature by different names as, vibro-thermography, thermo-sonics, sonic-IR, etc.
Accordingly, it is to be appreciated that while many NDE techniques have been used in the past for evaluation of heat damage in composites, most of them have limited success and capable of revealing gross damage only.